The present invention relates to the field of data communication electronics and to a wired or wireless signal transmitter or receiver and methods providing improved signal-to-noise ratio (SNR) performance for the reception of data packets transmitted in the form of symbols modulated onto a signal. More particularly, methods and devices are disclosed for detecting the preamble and symbol boundaries of received data packets encoded with OFDM and other multi-carrier symbol transmission systems. Also disclosed are combinations of sensitive preamble detection and data payload and/or signal field encoding methods to spread data redundantly, to improve reception under a low SNR condition. This invention may be applied to a variety of standards utilizing OFDM and similar technologies, including IEEE 802.11a, 802.11g, Hiperlan/2 and MMAC.
Orthogonal frequency division multiplexing (OFDM) is a spectrally efficient modulation scheme with applications in both wired and wireless communications. It is applied in existing wireless systems including IEEE 802.11a and 802.11g and is proposed for several next-generation wireless systems including IEEE 802.16, 4th generation cellular and digital television broadcasting. Because OFDM has high spectral efficiency, it is susceptible to various radio transmission impairment mechanisms.
Furthermore, the combination of data communications based on OFDM techniques with data protocols based on packets creates additional challenges. In particular, data protocols based on packets may include training preambles that a receiver detects and locks onto. Relatively brief preambles typically are specified to increase the relative payload capacity of a packet. Detecting and locking onto a short preamble is difficult because a receiver takes time to adapt to variations in the signal received. Inefficiency in detecting or locking onto a preamble can result in either missed or misinterpreted packets, leading to failure to establish communications, repeated retransmissions or a loss of signal. Efficiency can be expressed in terms of detecting and locking onto a preamble at a relatively low signal to-noise ratio (SNR), which implies a greater signal range or resistance to noise.
Methods and devices may be designed that facilitate detection of a transmitted data packet preamble at a lower SNR, which is desirable for more efficient data transmission. A system that takes advantage of the structure of a preamble and facilitates their detection at a lower SNR, could adapt the data payload transmission protocol based on the available SNR.
Accordingly, an opportunity arises to develop methods and apparatus that efficiently detect and lock onto a preamble and that adapt data transmission to the available SNR.